This invention relates to bidirectional fiber optic communication systems and, more particularly, to a transceiver assembly for use in such systems.
In a typical bidirectional fiber optic communication system, such as a fiber-in-the-loop (FITL) system, optical signals propagate in both directions over a single optical fiber. Terminal equipment at each end of the fiber includes a transceiver which launches an outgoing signal onto the fiber and which receives an incoming signal from the same fiber. The outgoing and incoming signals may have the same wavelength or different wavelengths. Generally, within the transceiver assembly the outgoing and incoming signals are directed over separate optical paths between the fiber and either a light source (e.g., a laser diode) and a light detector (e.g., a photodiode). Most designs incorporate a beam splitter which partially transmits the outgoing signal from the laser diode to an output fiber (e.g., a fiber pigtail) and partially reflects the incoming signal to the photodiode. The remaining portion of the outgoing signal is also reflected by the beam splitter to incidence upon various surfaces internal to the assembly. These surfaces further reflect or scatter the signal. (For simplicity, we will refer to these as "reflections".) Some of the reflections of the outgoing signal reach the photodiode and constitute crosstalk when the system is operated in a full duplex mode. That is, the photodiode is intended to detect only the incoming signal, not reflected portions of the outgoing signal. To the extent that the latter is detected, it constitutes noise or crosstalk. A need remains in the art for reducing such crosstalk. There is also a need to reduce crosstalk without significantly degrading coupling efficiency to the fiber.